New speed cameras are being tested in residential areas to catch drivers averaging more than 20mph.

They calculate a car's average speed within the 20mph zone and transmit the data to a processing centre.

The system is seen as an alternative to road humps and will catch drivers who slow down for conventional cameras and then speed up again.

It is being tested in Camden, north London, and Belfast but could be used to develop more 20mph zones.

The London trial, which runs until March 2007, involves two cameras installed at either end of Mansfield Road in Camden, which has a 20mph limit.

Environmentally friendly

A Camden Council spokesman said as this was a trial no penalties were being issued to speeding drivers.

The Specs cameras are synchronised but not physically linked, which avoids the need to dig up the roads between them to lay cables.

As a road safety initiative I give this scheme nought out of 10

Paul SmithSafe Speed road safety campaign

"In the future, [the cameras] could open up the possibility of enforcing speed across a residential area using only cameras installed at the entrance and exit junctions," said the council spokesman.

The cameras are also considered to be a more environmentally friendly option, as road humps are believed to push up vehicles' carbon monoxide emissions.

Paul Smith, of the Safe Speed road safety campaign, which believes cameras divert motorists' attention away from the roads, said the scheme would not be as effective as humps or other traffic calming measures.

He said because the cameras were intended to be at the entrance and exit of the zone, cars stopping or travelling within the area would not be monitored.

"As a road safety initiative I give this scheme nought out of 10," he said.

The technology will need Home Office approval - which is expected to be given later this year - before it can be rolled out.